Got one of the vintage Threshold pre. It's oldie but sounds very yummy. While I don't use it that much, I think it's the keeper till I kick the bucket.

The question is should I recap the FET1? It sounds good as is but it's probably about time for proper recapping and upgrading. I think it's time for at least two big filter electrolytic caps to get a retirement.

I don't have any manual or schema for it but it's not hard to do recapping. While I do recapping, what parts should be upgraded too?

Filter caps for sure. I would also replace all those 10uF polyesters with something cheap like Auricaps. Going overboard is probably not worth it - too many pots and old switches to make it truly transparent.

I'm also interested in this preamp.
I know the original fet one uses transformers for the MC amplification and the Series II a MC headamp.
Is anybody aware of other differences between the two?
Had this preamp myself 23 years ago but traded it in for a Levinson ML-7A.
Found this Japanes refurbishing link for the "Series I":Threshold FET1

Well they certainly cherished your products from that era!
I'm only wondering why they removed the grease (green stuff) from the Noble pots. Who did principal designing for these preamps (Fet One & Two) Eric Lauchli or you?

Okay that's sorted out now. Very generously of you that he could print his name also on the circuit board.
Can you recall why you switched from the Jensen transformers in Series I to the MC headamp in Series II?
Won't bother you any further about this subject.